In recent years, in order to increase the production of oil and gas, a fracturing technique has been widely used in which a fluid is injected under high pressure in rock forming oil and gas layers so as to break the rock and form cracks therein and, at the same time, a proppant (propping agent), such as coarse sand, is introduced to hold or maintain the cracks, thereby accelerating the production of oil and gas.
As a fluid for use in this fracturing, that is, a fracturing fluid, water with a polymer dissolved therein or with a polymer dissolved and cross-linked therein is mainly used. One of major functions required for the fluid is to have such rheological properties as to make it possible to form cracks in the reservoir and to convey a proppant thereinto. Another feature is that the fluid has only a small leak-off. As polymers for use in the fluid, guar gum and/or derivatives thereof, such as hydroxypropylguar, hydroxyethylguar and carboxymethylguar are mainly used.
Recently, commercially successful exploration of oil and gas is difficult unless the oil and gas are prospected in a deeper stratum. For this reason, the temperature of the stratum tends to rise and, in many cases, it reaches 90.degree. C. or more.
At such high temperatures, the guar gum and/or derivatives thereof used in the fracturing fluid are deteriorated or decomposed. This leads to a reduction in rheological properties of the fracturing operation and creates a serious obstacle to the fracturing operation. Accordingly, the inherent functions of the fracturing fluid can be satisfactorily obtained only if degradation (e.g., caused by decomposition) of its rheological properties can be prevented even at elevated temperatures.